Ionic electrolyte membrane structure, method for its production and solid oxide fuel cell making use of ionic electrolyte membrane structure

ABSTRACT

To provide an ionic electrolyte membrane structure that enables contact between the air pole and the fuel pole in which structure an edge face of the interface between an ion conducting layer and an ion non-conducting layer stands bare on a plane, an ionic electrolyte membrane structure which transmits ions only is made up of i) a substrate having a plurality of pores which have been made through the substrate in the thickness direction thereof and ii) a plurality of multi-layer membranes each comprising an ion conducting layer formed of an ion conductive material and an ion non-conducting layer formed of an ion non-conductive material which have alternately been formed in laminae a plurality of times on each inner wall surface of the pores of the substrate in such a way that the multi-layer membranes fill up the pores completely; the ions only being transmitted in the through direction by way of the multi-layer membranes provided on the inner wall surfaces of the pores.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an ionic electrolyte membrane structure, amethod for its production and a solid oxide fuel cell making use of theionic electrolyte membrane structure.

2. Description of the Related Art

As our living standard becomes higher, they use electrical fittings inmore various ways of living and come to prefer brighter rooms than ever,so that, correspondingly thereto, the consumption of electricity forlighting has been increasing. The consumption of electricity has alsobeen increasing with a rise in the Internet connections and digitalcommunications, making use of personal computers. However, in thepresent state of affairs, the construction of power plants or stationsdoes not always turn out as desired, and the situation is that, in orderto meet such increasing demand for electricity, the utilization ofreproducible energy such as solar-cell electricity has to be taken intoconsideration.

It is difficult in the present state of affairs to construct large-scalepower plants and also the loss in electricity during its transmissioncan not be ignored in conventional power generation and transmissionsystems. Accordingly, distributed (or on-site) power generation isconsidered to be of a great trend hereafter. A system in which solarcells are installed in homes to cover much of the home power consumptionis considered to come to be an important way. The utilization of fuelcells making use of town gas or the like is also considered to come tobe an important choice.

Now, there are various types in the fuel cells. In a polymer electrolytefuel cell (hereinafter often also “PEFC”), which is a prevalent fuelcell at present, it necessitates water as an ancillary mechanism for theconduction of protons (hydrogen ions), and hence it is restricted to itsuse at 100° C. or below only. Its actual use is said to be at 80° C. orbelow, and the fuel cell has been found to have a limitation on how theheat be utilized and have a problem on its lifetime because a thinpolymer layer is used as an electrolyte membrane.

Meanwhile, as a type considered optimal for users of medium-scaleelectricity as in homes, convenience stores and other various shops, asolid oxide fuel cell (hereinafter often also “SOFC”) is available. TheSOFC is basically made up of a solid electrolyte capable of transmittingions selectively and, provided respectively on both sides thereof, twoelectrodes (an air pole and a fuel pole) holding the former betweenthem. Then, oxygen is flowed through the air pole and hydrogen throughthe fuel pole whereby chemical reaction proceeds and electricity isgenerated. The electrolyte may at least be a material capable oftransmitting either of oxygen ions and hydrogen ions. Usually, in viewof restrictions on materials, the material capable of transmittingoxygen ions is used. Stabilized zirconia in which yttrium oxide is addedto zirconium oxide so as to make its structure stable (hereinafter oftenalso “YSZ”) is used as the electrolyte material. Lanthanum manganitehaving perovskite structure and part of lanthanum of which has beensubstituted with an alkaline earth metal, [La_(1-X)(M)_(X)]_(Y)MnO₃ (M:an alkaline earth metal), is used for the air pole, and as the fuelpole, nickel zirconia cermet is used which is prepared by mixing YSZwith a stated amount of Ni.

As structure of the SOFC, what is known is a structure in which, asshown in FIG. 1, a single cell 40 made up of a solid electrolyte 41 andprovided respectively on both sides thereof an air pole 42 and a fuelpole 43 is set in layers on an interconnector 44 having fuel passages 47and oxidizing agent passages 48 and the electrolyte of which is providedwith mechanical strength.

Such an SOFC is advantageous in view of lifetime because it makes use ofan inorganic material for the electrolyte, and can utilize heat invirtue of its high service temperature, where its total efficiency is50% or more, which exceeds the efficiency of about 35% the PEFC has. Inaddition, although the PEFC requires use of platinum as a catalyst,which is expensive, the SOFC, which is used at a high temperature of200° C. or above, does not at all require use of any catalyst such asplatinum, or makes it enough to use it in a smaller amount than any PEFCat least which is driven at room temperature. This is characteristic ofthe SOFC, thus the SOFC can be said to be superior in this respect aswell, and is sought to be put into practical use.

In the conventional SOFC, as stated above the YSZ or the like is used asthe electrolyte material and the conduction of oxygen ions is utilized,where, because of a low ionic conductivity, the fuel cell requires acertain degree of high temperature in order to secure any necessaryionic conductivity, and usually the electricity is generated at about800° C. However, such a high temperature as service temperature maycause a great temperature difference in the interior of the fuel cell,and it has sometimes come about that this temperature difference causesthe fuel cell to break due to a difference in thermal expansion.

As a countermeasure therefor, the SOFC must be made to slowly change intemperature over a period of many hours when it is started or stopped.Accordingly, the SOFC is considered not usable in homes and the likewhere its on-off is frequently repeated, and is considered suitable forits use in convenience stores and the like where the electricity iscontinuously used day and night. In addition, because of its use at ahigh temperature, it may break, besides its break due to the differencein thermal expansion, due to growth in size or change in shape ofparticles in the interior of its electrolyte membrane. Thus, in order tomake an SOFC with a high energy efficiency usable in a broader rangeinclusive of its use in general homes, it is deemed to be a subject, forthe above reasons, how the SOFC be made usable at a lower temperature.Hence, it is sought to develop a material having a high ionicconductivity even at 500° C. or below, or at much lower temperature,i.e., a temperature close to room temperature.

Now, as prior art, Non-patent Document 1 (J. Garcia-Barriocanal, A.Rivera-Calzada, M. Varela, Z. Sefrioui, E. Iborra, C. Leon, S. J.Pennycook, J. Santamarial; Science, 321 (2008) 676: “Colossal IonicConductivity At Interfaces Of Epitaxial ZrO₂: Y₂O₃/SrTiO₃Heterostructures”) discloses that, where the YSZ (8 mol % of Y₂O₃ ismixed) that has conventionally been available as an oxygen ionconductive material and an ion non-conductive material SrTiO₃(hereinafter often simply “STO”) are mutually layered, oxygen isconducted through the interface between the two layers and the fuel cellshows an ionic conductivity of as high as 1×10² S/cm even at about 200°C.

Usually, fuel cells are considered necessary for them to have an ionicconductivity of 1×10⁻² S/cm or more in a service temperature range, andhence, the above ionic conductivity of 1×10² S/cm can be said to be asufficient ionic conductivity. Also, in the double-layer membraneproposed as above, it follows that, as long as ions are to be conductedby equal distance, the amount of ionic conduction more increases as thenumber of interfaces is made larger as far as possible. Then, in theabove way of conduction, where such membranes are formed in laminae, theoxygen is conducted through the interior of interfaces between multiplelayers, and therefore the ions are conducted in parallel to themembranes formed in laminae. However, in usual fuel cell electrolytemembranes or ion separation membranes, ions must be conducted in thedirection perpendicular to the membranes, thus the double-layer membraneproposed as above is required to be structurally remedied in order forit to be put into practical use.

SUMMARY OF THE INVENTION

The present invention has been made taking note of such a problem.Accordingly, what it aims is to provide an ionic electrolyte membranestructure having a high ionic conductivity, a method for its productionand a solid oxide fuel cell making use of the ionic electrolyte membranestructure.

In order to apply the way of conduction disclosed in Non-patent Document1, an ion conducting layer composed of an ion conductive material andhaving a thickness of several atomic layers and an ion non-conductinglayer composed of an ion non-conductive material and having a thicknessof several atomic layers may be formed in laminae to make up a laminatedmembrane having a thickness in the order of centimeters, and thislaminated membrane may be cut in the direction perpendicular to theplane of the laminated membrane to obtain a membrane lamination crosssection that may facilitate ionic conduction, where ions may beconducted through between layers of this membrane lamination crosssection (i.e., through the interior of interfaces between ion conductinglayers and ion non-conducting layers), thus it may be possible to obtainan ionic electrolyte membrane having a high ionic conductivity. This,however, is impractical because it takes an astronomical time to form inlaminae the ion conducting layer having a thickness of several atomiclayers and the ion non-conducting layer having a thickness of severalatomic layers to make up the laminated membrane having a thickness inthe order of centimeters.

Accordingly, the present inventor has made extensive studies, and, as aresult thereof, has discovered that an edge face of the interfacebetween an ion conducting layer and an ion non-conducting layer (whichedge face corresponds to the above membrane lamination cross section)may be made to stand bare on a plane without employing such a cuttingmethod as above, to obtain an ionic electrolyte membrane structure thatenables contact between the air pole and the fuel pole. Thus, theinventor has accomplished the present invention.

That is, the present invention first provides as a first embodiment anionic electrolyte membrane structure which transmits ions only,comprising:

a substrate having a plurality of pores which have been made through thesubstrate in the thickness direction thereof; and

a plurality of multi-layer membranes each comprising an ion conductinglayer formed of an ion conductive material and an ion non-conductinglayer formed of an ion non-conductive material which have alternatelybeen formed in laminae a plurality of times on each inner wall surfaceof the pores of the substrate in such a way that the multi-layermembranes fill up the pores completely;

the ions only being transmitted in the through direction by way of themulti-layer membranes provided on the inner wall surfaces of the pores.

The present invention also second provides as a second embodiment anionic electrolyte membrane structure which transmits ions only,comprising:

a united multi-layer membrane comprising an ion conducting layer formedof an ion conductive material and an ion non-conducting layer formed ofan ion non-conductive material which have alternately been formed inlaminae a plurality of times;

the united multi-layer membrane having been formed by:

bonding a substrate on one side thereof to a holding plate; thesubstrate being the substrate in the ionic electrolyte membranestructure according to the first embodiment, having the multi-layermembranes filling up the pores completely;

removing the substrate by dissolution to leave columnar multi-layermembranes to provide between the columnar multi-layer membranes an emptyspace extending through in the thickness direction of the structure tobe formed; and

alternately forming in laminae a plurality of times an ion conductinglayer formed of an ion conductive material and an ion non-conductinglayer formed of an ion non-conductive material on the inner wall surfaceof the empty space defined substantially by the columnar multi-layermembranes, serving as first multi-layer membranes, to form a secondmulti-layer membrane which fills up the empty space completely; thefirst multi-layer membranes and second multi-layer membrane having beenunited to form the united multi-layer membrane;

the ions only being transmitted in the through direction by way of theunited multi-layer membrane formed.

The present invention still also third provides a method for producingthe ionic electrolyte membrane structure according to the firstembodiment, comprising:

on each inner wall surface of pores of a substrate having a plurality ofpores which have been made through the substrate in the thicknessdirection thereof, alternately forming in laminae a plurality of timesan ion conducting layer formed of an ion conductive material and an ionnon-conducting layer formed of an ion non-conductive material, by atomiclayer deposition (ALD) to form a plurality of multi-layer membraneswhich fill up the pores completely.

The present invention further fourth provides a method for producing theionic electrolyte membrane structure according to the second embodiment,comprising:

bonding a substrate on one side thereof to a holding plate; thesubstrate being the substrate in the ionic electrolyte membranestructure according to the first embodiment;

removing the substrate by dissolution to leave columnar multi-layermembranes to provide between the columnar multi-layer membranes an emptyspace extending through in the thickness direction of the structure tobe formed; and

alternately forming in laminae a plurality of times an ion conductinglayer formed of an ion conductive material and an ion non-conductinglayer formed of an ion non-conductive material, by atomic layerdeposition (ALD) on the inner wall surface of the empty space definedsubstantially by the columnar multi-layer membranes, serving as firstmulti-layer membranes, to form a second multi-layer membrane which fillsup the empty space completely; the first multi-layer membranes andsecond multi-layer membrane having been united to form a unitedmulti-layer membrane.

The present invention still further fifth provides a solid oxide fuelcell comprising a solid electrolyte capable of transmitting ionsselectively, and an air pole provided on one side of the solidelectrolyte and a fuel pole provided on the other side thereof, wherein;

the solid electrolyte comprises the ionic electrolyte membrane structureaccording to the above first embodiment or second embodiment.

Thus, the ionic electrolyte membrane structure according to the firstembodiment comprises i) a substrate having a plurality of pores whichhave been made through the substrate in the thickness direction thereofand ii) a plurality of multi-layer membranes each comprising an ionconducting layer formed of an ion conductive material and an ionnon-conducting layer formed of an ion non-conductive material which havealternately been formed in laminae a plurality of times on each innerwall surface of the pores of the substrate in such a way that themulti-layer membranes fill up the pores completely. The ions only aretransmitted in the through direction, i.e., in the directionperpendicular to electrodes formed on both sides of the ionicelectrolyte membrane, by way of the multi-layer membranes provided onthe inner wall surfaces of the pores.

The ionic electrolyte membrane structure according to the secondembodiment comprises a united multi-layer membrane comprising an ionconducting layer formed of an ion conductive material and an ionnon-conducting layer formed of an ion non-conductive material which havealternately been formed in laminae a plurality of times. The unitedmulti-layer membrane has been formed by i) bonding a substrate on oneside thereof to a holding plate; the substrate being the substrate inthe ionic electrolyte membrane structure according to the firstembodiment, having the multi-layer membranes filling up the porescompletely, ii) removing the substrate by dissolution to leave columnarmulti-layer membranes to provide between the columnar multi-layermembranes an empty space extending through in the thickness direction ofthe structure to be formed, and iii) alternately forming in laminae aplurality of times an ion conducting layer formed of an ion conductivematerial and an ion non-conducting layer formed of an ion non-conductivematerial, by atomic layer deposition (ALD) on the inner wall surface ofthe empty space defined substantially by the columnar multi-layermembranes, serving as first multi-layer membranes, to form a secondmulti-layer membrane which fills up the empty space completely; thefirst multi-layer membranes and second multi-layer membrane having beenunited to form the united multi-layer membrane. The ions only aretransmitted in the through direction, i.e., in the directionperpendicular to electrodes formed on both surfaces of the ionicelectrolyte membrane structure, by way of the united multi-layermembrane formed.

Thus, these ionic electrolyte membrane structures are so structured thatthe edge faces of the interfaces between ion conducting layers and ionnon-conducting layers which have alternately been formed in laminae onthe pore inner wall surfaces or on the previous pore inner wall surfacesand the inner wall surface of the empty space defined substantially bythe columnar multi-layer membranes stand bare on a plane, where the ionsare transmitted in the through direction of the pores which have beenmade through the substrate or of the pores which had been made throughthe substrate and the space which had extended through between thecolumnar multi-layer membranes. Hence, the edge faces of the interfacesbetween ion conducting layers and ion non-conducting layers can be madeinto contact with the air pole and the fuel pole.

Accordingly, the use of any of these ionic electrolyte membranestructures in the SOFC enables the SOFC to be usable at a lowertemperature, on account of its high ionic conductivity, so that thepresent ionic electrolyte membrane structure can be effective in thatthe range in which the SOFC with a superior energy efficiency is usablecan be enlarged to a broader range inclusive of its use in generalhomes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic exploded perspective view of a conventional ionicelectrolyte membrane structure (SOFC).

FIG. 2 is a graphical representation showing the relationship betweenionic conductivity and temperature that is found when a laminatedmembrane in which YSZ (8 mol % of yttrium oxide is mixed) is used as anoxygen ion conductive material and STO (SrTiO₃) as an ion non-conductivematerial is formed in different layer thicknesses. A graphicalrepresentation at the upper column in the FIG. 2 graphicalrepresentation shows the relationship between the number of layers ofthe laminated membrane, n_(i), and the ionic conductivity. A graphicalrepresentation at the lower column in the FIG. 2 graphicalrepresentation shows the relationship between the layer thickness of thelaminated membrane, t_(YSZ) and the ionic conductivity.

FIG. 3 is a schematic perspective view showing the structure of aconventional laminated membrane in which an ion conducting layer formedof YSZ and an ion non-conducting layer formed of STO are formed inlaminae on a flat-plate substrate (not shown).

FIG. 4 is a schematic perspective view of a membrane lamination crosssection obtained when the laminated membrane shown in FIG. 3 is cut inthe direction perpendicular to the plane of that membrane.

FIG. 5 is a schematic perspective view of a substrate used in thepresent invention and having a plurality of pores which have been madethrough the substrate in its thickness direction.

FIG. 6 is a schematic sectional perspective view of a partially enlargedpart of the ionic electrolyte membrane structure according to the firstembodiment of the present invention which structure is in the course ofproduction (at a stage just before the pores of the substrate havecompletely been filled up with ion conducting layers and ionnon-conducting layers).

FIG. 7 is a schematic sectional perspective view of a partially enlargedpart of the ionic electrolyte membrane structure according to the firstembodiment of the present invention (Example 1) in which structure thepores of the substrate have completely been filled up with ionconducting layers and ion non-conducting layers.

FIG. 8 is a schematic sectional perspective view of a partially enlargedpart of the ionic electrolyte membrane structure according to the secondembodiment of the present invention which structure is in the course ofproduction (at a stage where the substrate of the ionic electrolytemembrane structure according to the first embodiment of the presentinvention has been removed to provide an empty space between columnarmulti-layer membranes on a holding plate).

FIG. 9 is a schematic sectional perspective view of a partially enlargedpart of the ionic electrolyte membrane structure according to the secondembodiment of the present invention in which structure the empty spacebetween columnar multi-layer membranes on a holding plate has completelybeen filled up with ion conducting layers and ion non-conducting layers.

FIG. 10 is a graphical representation showing electricity generationcharacteristics at 200° C. of a solid oxide fuel cell according toExample 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described below in detail with reference to theaccompanying drawings.

(1) Ion conductive material and ion non-conductive material:

In Non-patent Document 1, it discloses a laminated membrane in which YSZ(8 mol % of yttrium oxide Y₂O₃ is mixed) is used as an oxygen ionconductive material and STO (SrTiO₃) as an ion non-conductive material.

Its ionic conductivity measured when the laminated membrane is formed indifferent layer thicknesses has been examined, and results obtained areshown in the FIG. 2 graphical representation. The relationship betweenthe number of layers of the laminated membrane and the ionicconductivity and the relationship between the layer thickness of thelaminated membrane and the ionic conductivity have also been examined,and the relationship between the number of layers of the laminatedmembrane, n₁, and the ionic conductivity and the relationship betweenthe layer thickness of the laminated membrane, t_(YSZ), and the ionicconductivity are shown at the upper column and at the lower column,respectively, in the FIG. 2 graphical representation.

From the graphical representation showing “the relationship between thenumber of layers of the laminated membrane, n_(i), and the ionicconductivity” shown at the upper column in the FIG. 2 graphicalrepresentation, it is seen that;

(i) the level of oxygen ionic conduction increases with an increase inthe number of layers of the laminated membrane, and is proportional tothe number of layers of the laminated membrane.

From the graphical representation showing “the relationship between thelayer thickness of the laminated and the ionic conductivity” shown atthe lower column in the FIG. 2 graphical representation, it is also seenthat;

(ii) the level of oxygen ionic conduction does not depend on the layerthickness of the laminated membrane.

From the groups of data of layer thicknesses 1 nm, 5 nm, 20 nm, 30 nmand 62 nm in the FIG. 2 graphical representation (see marks shown bysquares, triangles and so forth), it is still also seen that;

(iii) the oxygen ionic conductivity is shown as ionic conductivityσ>1×10 S/cm in approximation, in the vicinity of T=200° C.(1,000/T=2.1).

In general, fuel cells are considered necessary for them to have anionic conductivity of σ>1×10⁻² S/cm or more in solid electrolytes. Also,NAFION (trade name; available from Sigma-Aldrich Corporation), which isa proton (hydrogen ion) conducting membrane well-known as the PEFC, isknown to have an ionic conductivity of 1×10⁻² to 3×10⁻² S/cm at atemperature between room temperature and 80° C.

Thus, the value of conductivity itself of the ion conductive materialdisclosed in Non-patent Document 1 can be said to be well attractive.However, the path of ionic conduction is bound to be the border (i.e.,boundary or interface) between the YSZ layer and the STO layer. Hence,in a usual membrane forming method, a laminated membrane is obtained ona flat-plate substrate (not shown) which membrane is so structured as tobe in parallel to the plane of this substrate, as shown in FIG. 3. Aslong as it is the laminated membrane structured as shown in FIG. 3, itfollows that the border (i.e., boundary or interface) between each YSZlayer and each STO layer is present in parallel to the laminatedmembrane and the ions flow only in the direction parallel to thelaminated membrane.

In any of fuel cell electrolyte membranes, ion sensors and ionseparation membranes where ionic conduction is utilized, the directionof ionic conduction must be the direction perpendicular to electrodes tobe connected to the ionic electrolyte membrane. Accordingly, as long asthe laminated membrane structured as shown in FIG. 3 is used, it comesnecessary that the YSZ layer and the STO layer are formed in laminae tomake up a laminated membrane having a thickness in the order ofcentimeters and this laminated membrane is cut in the directionperpendicular to the plane of the laminated membrane so that the borders(i.e., boundaries or interfaces) between YSZ layers and STO layers canbe connected to an electrode on their cross sections as shown in FIG. 4.It, however, is impractical because it takes an astronomical time toform the YSZ layer and STO layer in laminae until they come to be innecessary thickness. In addition, it is desirable for the ionicelectrolyte membrane to be as small as possible in layer thickness, andit is considered impossible for such a method as above to form the ionicelectrolyte membrane in a thickness in the order of microns.

(2) Ionic Electrolyte Membrane Structure:

The ionic electrolyte membrane structure according to the firstembodiment of the present invention is constituted of i) a substratehaving a plurality of pores which have been made through the substratein the thickness direction thereof as shown in FIG. 5 and ii) aplurality of multi-layer membranes each comprising an ion conductinglayer formed of an ion conductive material and an ion non-conductinglayer formed of an ion non-conductive material which have alternatelybeen formed in laminae a plurality of times on each inner wall surfaceof the pores of the substrate as shown in FIG. 6. These layers arealternately so formed in laminae that the multi-layer membranes fill upthe pores completely (see FIG. 7). The ions only are transmitted in thethrough direction by way of the multi-layer membranes provided on theinner wall surfaces of the pores.

The ionic electrolyte membrane structure according to the secondembodiment of the present invention is constituted of a unitedmulti-layer membrane comprising an ion conducting layer formed of an ionconductive material and an ion non-conducting layer formed of an ionnon-conductive material which have alternately been formed in laminae aplurality of times. This united multi-layer membrane has been formed inthe following way:

i) bonding a substrate on one side thereof to a holding plate. Thesubstrate is the substrate in the ionic electrolyte membrane structureaccording to the first embodiment, having the multi-layer membranesfilling up the pores completely (i.e., the substrate in an ionicelectrolyte membrane structure comprising i) a substrate having aplurality of pores which have been made through the substrate in thethickness direction thereof and ii) a plurality of multi-layer membraneseach comprising an ion conducting layer formed of an ion conductivematerial and an ion non-conducting layer formed of an ion non-conductivematerial which have alternately been formed in laminae a plurality oftimes on each inner wall surface of the pores of the substrate in such away that the multi-layer membranes fill up the pores completely);

ii) thereafter removing the substrate by dissolution to leave columnarmulti-layer membranes to provide between the columnar multi-layermembranes an empty space extending through in the thickness direction ofthe structure to be formed (i.e., an empty space formed between thecolumnar multi-layer membranes on the holding plate after the substrateportion has been removed, as shown in FIG. 8); and

iii) thereafter alternately forming in laminae a plurality of times anion conducting layer formed of an ion conductive material and an ionnon-conducting layer formed of an ion non-conductive material on theinner wall surface of the empty space defined substantially by thecolumnar multi-layer membranes, serving as first multi-layer membranes,to form a second multi-layer membrane which fills up the empty spacecompletely (see FIG. 9). The first multi-layer membranes and secondmulti-layer membrane have been united to form the united multi-layermembrane.

The ions only are transmitted in the through direction by way of theunited multi-layer membrane formed.

The ionic electrolyte membrane structure is described below in detail.

(2-a) Substrate Having Pores:

In the substrate having a plurality of pores, i.e., the substrate havinga plurality of pores which have been made through the substrate in itsthickness direction, the pores are through holes and their size, periodand so forth may artificially be controlled by nanoimprinting. As anexample, an anodized aluminum substrate is described which may be usedas the substrate. Aluminum may be anodized by, e.g., setting an aluminumplate as the anode, connecting carbon to the cathode and applying avoltage of approximately from several volts (V) to tens of volts (V) inthe state both the electrodes are immersed in acid such as oxalic acid,where the surface of the aluminum plate is acceleratedly oxidized andpores of tens of nanometers (nm) in diameter are formed inself-alignment, vertically to the plate surface and in a depth in theorder of millimeters (mm). Thereafter, setting the electrodes opposite,a voltage may be applied, where hydrogen gas is generated at theinterface between the aluminum portion and the pore-formed portion, sothat an oxide film can readily be peeled from the metallic surface. Asan example, which depends on the voltage and the type of acid, asubstrate can be obtained in which the through holes (pores) having sizeas shown in FIG. 5 (50 nm each in diameter and 100 nm incenter-to-center distance) are formed in self-alignment.

Then, the ion conducting layer and the ion non-conducting layer arealternately formed in laminae on each inner wall surface of the pores,where, when each pore is 100 nm to 200 nm in diameter for example, thewhole membrane formed as a laminated membrane is required to be about 50to about 100 nm in thickness as measured from the center of each pore.Even where the size of area that is necessary for the laminated membraneitself is in the order of square centimeters, it follows that the timetaken for the above layers to be formed in laminae on the inner wallsurfaces of the pores standing in a large number through the substratein its thickness direction can enough be a time for forming membranes ofabout 100 nm or about 200 nm. Thus, this can be a realistic andpractical method. In the case when the aluminum plate is anodized, thearea ratio found between the pore portions and the substrate portion(non-pore portion) on the top surface of the substrate is substrateportion (non-pore portion): pore portions=about 55:45. In the event thatthe ion conducting layer and the ion non-conducting layer arealternately formed in laminae on the pore inner wall surfaces until themulti-layer membranes fill up the pores completely, the area of an ionicconduction region comes to 45% of the whole top surface area of thesubstrate. That is, ionic conductivity with respect to the whole topsurface of the substrate comes down to only about a half. Where thesubstrate in its whole top surface area is an ionic conductor at about200° C. and its ionic conductivity is 1×10² S/cm, the former's ionicconductivity comes to only about 5×10 S/cm. Loss in ionic conductivityto such an extent does not matter, and it can be said that asufficiently high ionic conductivity should have been secured.

(2-b) Oxygen Ion Conductive Material:

The oxygen ion conductive material making up an oxygen ion conductinglayer may include at least one selected from YSZ (zirconium oxidestabilized with yttrium oxide), LaGaO₃, CeO₂, SrFeO₃—, and SrCoO₃—. TheYSZ is a typical one, but has a difficult point in that its workingtemperature is a high temperature of about 800° C. As a substitutetherefor, a perovskite material typified by that of an LaGaO₃ series maybe used. What is herein termed as “series” refers to the one in which,in place of La, it has partly been substituted with Sr or the one the Gaof which has partly been substituted with Mg or the like. Statedspecifically, a perovskite series such as an LaScO₃ series is available,as exemplified by (La,Sr)ScO₃ and SmSrCoO₃. A CeO₂ series material isalso a preferable material. Stated specifically, it may include(Ce,Gd)O₂. In place of a tetravalent material of the perovskite oxide, atrivalent material may also be doped as an acceptor in a proportion of10% or less so as to be used as the following proton (hydrogen ion)conductive material.

(2-c) Proton (Hydrogen Ion) Conductive Material:

It is also preferable for the ion conducting layer to be formed of aproton (hydrogen ion) conductive material. The proton conductivematerial may preferably be a proton conductive material having aperovskite structure, and may preferably be at least one materialselected from BaCeO₃, SrCeO₃, BaZrO₃ and CeO₂. In the BaCeO₃, it alsoincludes one in which, in place of Ba, its part or the whole thereof hasbeen substituted with Sr or Zr and part of Ce has been substituted withZr or Y. The CeO₂ is also typical as the proton conductive material,which includes one in which part or the whole of Ce has been substitutedwith a rare earth element such as Sm.

(2-d) ALD (Atomic Layer Deposition):

The substrate used in the present invention has, as described above, aplurality of pores which have been made through the substrate in itsthickness direction. The pores each have a diameter of from about 100 nmto about 200 nm, and the substrate has a thickness corresponding to thethickness the electrolyte has. Here, the resistance value of substantialionic conduction decreases with an increase in thickness of theelectrolyte, and hence the ionic conductor may preferably be smaller inthickness. If, however, its thickness is too small, there is apossibility that such membrane imperfectness unwantedly makes the fuel(such as hydrogen) and the oxygen come mixed directly, and hence itsthickness must be so set that strength, reliability and conductivity maycompromise with one another in accordance with the extent of proficiencyin how to make it. In FIG. 5, the pore diameter is shown as 50 nm andthe thickness as 50 μm. In actuality, it is preferable for the thicknessto be set much smaller. As a method by which a membrane with a layerthickness of 10 nm or less is formed in laminae on the inner wallsurfaces of the pores having such a large aspect ratio (ratio of porelength or depth to pore diameter), ALD (atomic layer deposition) iseffective.

The ALD is a type of CVD processes, and is a process in which asubstrate member is placed in a vacuum vessel (film forming system), araw-material gas containing elements making up a molecular layer are fedinto the vacuum vessel and the molecular layer is formed by the reactiontaking place between molecules having adsorbed on the substrate surfaceor pore inner wall surfaces and a raw-material gas fed subsequently.This enables the layer thickness of the molecular layer to be controlledat the level of atomic layers, and is an optimum method for forminglaminated membranes on the pore inner wall surfaces. Then, in the filmforming system (atomic layer deposition system) used in the ALD, it isunnecessary to provide any expensive component part units, high-speedrotating mechanism and so forth which have been necessary in filmforming systems used in PVD and CVD. Thus, this enables cost reductionfor film formation, compared with conventional film forming processes.

In the method of forming a multi-layer membrane by ALD, a molecularlayer for each of some kinds of substances different in values ofphysical properties is formed in laminae on the substrate surface orpore inner wall surfaces to form a thin membrane having the desiredvalues of physical properties, and such a basic step is repeated aplurality of times to form a multi-layer membrane made up of some kindsof thin membranes. Then, in forming the respective thin membranes,raw-material gasses containing elements which are to make up respectivemolecular layers are alternately fed into the vacuum vessel (filmforming system), and the number of times for replacing the raw-materialgasses is controlled to make the respective thin membranes changecontinuously in composite values of physical properties.

In the ALD, many kinds of oxide layers or compound oxide layers likeSiO₂, Al₂O₅, Ta₂O₅ and/or TiO₂ and/or nitride layers can be formed.Also, different substances may be deposited for several atomic layers tomake a layer having new physical properties.

Where the ALD is used to form, e.g., an Al₂O₃ mono-atomic layer(monomolecular layer), the process is completed through the followingfour steps.

(1) Water molecules are introduced to make OH groups adsorbed on thesubstrate surface or pore inner wall surfaces or on any surface(s) onwhich a membrane or membranes has or have already been formed.

(Reaction for First Layer Downward)

2H₂O+:O—Al(CH₃)₂→: Al—O—Al(CH)₂+2CH₄

(2) Any Excess Water Molecules are Purged to Effect Evacuation.

(3) TMA [trimethyl aluminum: Al(CH₃)₃] gas, a raw-material gas for theAl₂O₃ membrane, is introduced. TMA molecules react with the OH groups togenerate CH₄ gas.

(Reaction for First Layer)

Al(CH₃)₃+:O—H→:O—Al(CH₃)₂+CH₄

(Reaction for First Layer Downward)

Al(CH₃)₃+:Al—O—H→:Al—O—Al(CH₃)₂+CH₄

(4) The CH₄ Gas Purged to Effect Evacuation.

Through these four steps, an Al₂O₃ membrane of about 0.1 nm is formed,and hence the above four steps are repeated to increase its layerthickness until it comes to have the desired layer thickness.

In the method for producing the ionic electrolyte membrane structureaccording to the first embodiment of the present invention, the ionconducting layer formed of an ion conductive material and the ionnon-conducting layer formed of an ion non-conductive material arealternately formed in laminae a plurality of times by the ALD on eachinner wall surface of the holes of the substrate having a plurality ofpores which have been made through the substrate in its thicknessdirection, to form the multi-layer membranes which fill up the porescompletely with ion conducting layers and ion non-conducting layers, asshown in FIG. 7. Thus, the ionic electrolyte membrane structureaccording to the first embodiment of the present invention is obtainedin which the ions only are transmitted in the through direction by wayof the multi-layer membranes thus formed.

Incidentally, in the ionic electrolyte membrane structure according tothe first embodiment of the present invention, obtained by using theALD, the multi-layer membranes made up of ion conducting layers and ionnon-conducting layers stand formed not only on the pore inner wallsurfaces but also on the plane surface of the substrate. Accordingly, inorder to connect the resultant ionic electrolyte membrane structure toelectrodes (the air pole and the fuel pole), the multi-layer membraneformed on the plane surface of the substrate must be removed by amechanical means, e.g., by grinding to make the edge faces of theinterfaces between ion conducting layers and ion non-conducting layersstand bare to the surface.

Here, as the ion conductive material, the oxygen ion conductive materialor proton (hydrogen ion) conductive material described above may beused.

As the ion non-conductive material, usable are oxides of various types,such as STO (SrTiO₃). It may appropriately be selected taking account ofan affinity for the above ion conductive material, i.e., taking accountof coefficient of thermal expansion, lattice constant, crystal structureand so forth.

Now, in the ionic electrolyte membrane structure according to the firstembodiment of the present invention, what is utilized is that the ionsare conducted through the interfaces between layers of the multi-layermembranes provided on the pore inner wall surfaces and through the ionconducting layers of the same. Here, what should most direct attentionto is that, if, in the course the ionic electrolyte membrane structureis produced, any new fine through holes are formed in the substrate orthe pore inner wall surfaces are not completely filled up with themulti-layer membranes, a difficulty may arise such that the fuel gas andthe oxygen gas come directly mixed when the ionic electrolyte membranestructure is set in a fuel cell. Accordingly, after the pore inner wallsurfaces have been judged to have completely been filled up with themulti-layer membranes, it is desirable to form one ion conducting layeras the last one membrane, taking account of such a difficulty.

Next, the ionic electrolyte membrane structure according to the secondembodiment of the present invention may be produced in the followingway, by using the ALD.

The substrate is, on one side thereof, bonded to a holding plate; thesubstrate being that in the ionic electrolyte membrane structureaccording to the first embodiment of the present invention as describedabove, made up of i) the substrate having a plurality of pores whichhave been made through the substrate in its thickness direction and ii)the multi-layer membranes each comprising the ion conducting layer andthe ion non-conducting layer which have alternately been formed inlaminae a plurality of times on each inner wall surface of the pores ofthe substrate in such a way that the multi-layer membranes fill up thepores completely. Then, preferably after the multi-layer membrane formedinevitably on the plane surface of the substrate in the ionicelectrolyte membrane structure of the first embodiment has been removedby a mechanical means, e.g., by grinding, the substrate portion isremoved by dissolution to leave columnar multi-layer membranes toprovide between the columnar multi-layer membranes an empty space (i.e.,an empty space formed between the columnar multi-layer membranes on theholding plate after the substrate portion has been removed) extendingthrough in the thickness direction of the structure to be formed, asshown in FIG. 8.

Next, the ion conducting layer formed of an ion conductive material andthe ion non-conducting layer formed of an ion non-conductive materialare alternately formed in laminae a plurality of times as shown in FIG.9, by the ALD on the inner wall surface of the empty space definedsubstantially by the columnar multi-layer membranes, serving as firstmulti-layer membranes, to form a second multi-layer membrane which fillsup the empty space completely. Thus, the first multi-layer membranes andsecond multi-layer membrane have been united to form the unitedmulti-layer membrane to obtain the ionic electrolyte membrane structurein which the ions only are transmitted in the through direction by wayof the united multi-layer membrane formed.

Incidentally, as stated previously, in the ionic electrolyte membranestructure according to the first embodiment of the present invention,obtained by using the ALD, the multi-layer membranes made up of ionconducting layers and ion non-conducting layers stand formed not only onthe pore inner wall surfaces but also on the plane surface of thesubstrate. Accordingly, it is preferable that the multi-layer membraneformed on the plane surface of the substrate in the ionic electrolytemembrane structure of the first embodiment is removed by a mechanicalmeans, e.g., by grinding, to make the plane surface of the substratestand bare to the surface, thereafter the substrate portion (aluminaportion) is removed by dissolution using a chromium-phosphoric acidliquid mixture or the like to provide the empty space extending throughin the thickness direction of the structure to be formed andsubsequently the above ALD is carried out.

In the ionic electrolyte membrane structure according to the secondembodiment of the present invention, in which the ions are transmittedin the through direction by way of the united multi-layer membrane madeup of the first multi-layer membranes and second multi-layer membrane,as compared with the ionic electrolyte membrane structure according tothe first embodiment of the present invention, in which the ions aretransmitted in the through direction by way of only the multi-layermembranes provided on the pore inner wall surfaces, the secondmulti-layer membrane provided on the inner wall surface of the emptyspace formed by removing the substrate portion is also utilized. Invirtue of such an additional portion, the service area as theelectrolyte membrane comes to 100%. Thus, different from the ionicelectrolyte membrane structure according to the first embodiment of thepresent invention, it in principle does not come about that the value ofionic conductivity comes small.

In the ionic electrolyte membrane structure according to the secondembodiment of the present invention, too, in order to connect it toelectrodes (the air pole and the fuel pole), the edge faces of theinterfaces of the columnar first multi-layer membranes, which had beenprovided on the pore inner wall surfaces, and the edge faces of theinterfaces between layers of the second multi-layer membrane provided onthe inner wall surface of the empty space defined substantially by thecolumnar multi-layer membranes must be made to stand bare to thesurface.

In the ionic electrolyte membrane structures according to the firstembodiment and second embodiment of the present invention, it ispreferable that each layer of the ion conducting layers and ionnon-conducting layers formed respectively on the pore inner wallsurfaces or on the previous hole inner wall surfaces and the inner wallsurface of the empty space defined substantially by the columnarmulti-layer membranes is in a layer thickness of from 1 atomic layer ormore to 10 nm or less. As long as it is 10 nm or less, the number of theinterfaces between ion conducting layers and ion non-conducting layerscan be made large as being preferable. Here, where the substrate usedhas pores of about 1 pm in diameter, the layer thickness of multi-layermembranes each is required to correspond to the pore diameter after all.Formation of such multi-layer membranes takes too long time to berealistic and practical. Hence, it is preferable for the pores to beabout 100 nm in diameter. In this case, conduction paths increase withan increase in the number of the membrane to be formed in laminae, andhence it is desirable for the layer thickness of each membrane of themulti-layer membrane to be not more than 10 nm. This is because, if thelayer thickness of each membrane is more than 10 nm, the number of themembrane to be formed in laminae decreases correspondingly and thenumber of interfaces decreases, resulting in a smaller number of theconduction paths.

(3) Solid Oxide Fuel Cell (SOFC):

The solid oxide fuel cell (SOFC) according to the present inventioncomprises a solid electrolyte capable of transmitting ions selectively,and an air pole provided on one side of the solid electrolyte and a fuelpole provided on the other side thereof, and is characterized in thatthe solid electrolyte comprises the ionic electrolyte membrane structureaccording to the first embodiment or second embodiment of the presentinvention, described above.

The ionic electrolyte membrane structure according to the firstembodiment or second embodiment of the present invention is sostructured that the edge faces of the interfaces between ion conductinglayers and ion non-conducting layers which have alternately been formedin laminae on the pore inner wall surfaces or on the previous pore innerwall surfaces and the inner wall surface of the empty space definedsubstantially by the columnar multi-layer membranes stand bare on aplane, where the ions are transmitted in the through direction of thepores which have been made through the substrate or of the pores whichhad been made through the substrate and the space which had extendedthrough between the columnar multi-layer membranes. Hence, the edgefaces of the interfaces between ion conducting layers and ionnon-conducting layers can be made into contact with the air pole and thefuel pole.

Accordingly, in the solid oxide fuel cell (SOFC) having the ionicelectrolyte membrane structure according to the first embodiment orsecond embodiment of the present invention, the SOFC is usable at alower temperature in virtue of the high ionic conductivity the ionicelectrolyte membrane structure has, so that the range in which the SOFCwith a superior energy efficiency is usable can be enlarged to a broaderrange inclusive of its use in general homes.

The present invention is described below in greater detail by givingExamples.

Example 1

A 30 μm thick alumina substrate obtained by anodizing an aluminum plateand having pores (100 nm in diameter) as through holes in the thicknessdirection of the substrate, standing open over the whole surfacethereof, was used as a substrate of an ionic electrolyte membranestructure according to Example 1. Then, YSZ (8 mol % of Y₂O₃ was mixed)was used as an ion conductive material and STO (SrTiO₃) as an ionnon-conductive material, and the YSZ and the STO were alternatelydeposited in laminae by ADL on each inner wall surface of the pores insuch a way that the layers were each 5 nm in thickness and themulti-layer membranes thus formed filled up the pores completely.

Stated more specifically, the alumina substrate was fastened onto astand with a heater, placed in a vacuum chamber, and the aluminasubstrate was kept at 250° C. while evacuating the inside of the vacuumchamber.

In the course of the foregoing, a raw material for Y₂O₃ (yttriumtrimethylcyclopentadienyl: YCpMe₃ as reaction product A) was mixed witha raw material for Zr₂O₃ [zirconium tetratertiary butoxide: Zr(O^(t)Bu)₄as reaction product B] in a molar ratio of 8% based on the latter, andthese were fed into the vacuum chamber for 100 mili-second to form a YSZmembrane. Thereafter, a purging gas shown in Table 1 was fed into thevacuum chamber for 2 seconds to purge any unreacted gas remaining at thefirst stage. Thereafter, the above step of forming the YSZ membrane wasrepeatedly carried out by 10 times in total to make the membrane have athickness of about 5 nm.

Next, strontium dicyclopentadenyltripropyl [Sr(CpPr₃)₂ as reactionproduct A] was mixed with titanium methoxide [Ti(OMe)₄ as reactionproduct B] and these were fed into the vacuum chamber at a flow rate of100 m/second to form an STO membrane. Thereafter, a purging gas shown inTable 1 was fed into the vacuum chamber for 2 seconds to purge anyunreacted gas remaining at the second stage. This step was repeatedlycarried out by 10 times in total to make the membrane have a thicknessof about 5 nm.

The foregoing steps as a whole were taken as one set, and repeatedlycarried out by five sets in total to form the YSZ membrane and the STOmembrane in laminae on the inner wall surfaces of the above pores of 100nm in diameter substantially in parallel thereto and in the form ofconcentric circles. These steps were completed at the time themulti-layer membranes fill up the pores completely.

The multi-layer membrane product was, on its substrate both sides,surface-abraded by reverse sputtering until the edge faces of interfacesof the multi-layer membranes came to appear which were formed in laminaeon the inner wall surfaces of the pores of 100 nm in diameter in theform of concentric circles, to obtain the ionic electrolyte membranestructure according to Example 1.

The ionic electrolyte membrane structure according to Example 1 thusobtained is diagrammatically shown in FIG. 7. The raw-material gases forthe ion conductive material and ion non-conductive material and thepurging gases as used in the ALD are also shown in Table 1 below.

TABLE 1 Reaction Purging Reaction Purging product A gas product B gasYSZ YCpMe₃ N₂ Zr (O^(t)Bu)₄ N₂ SrTiO₃ Sr (CpPr₃)₂ N₂ Ti (OMe)₄ N₂Here, Cp: a cyclopentadienyl;

(O^(t)Bu)₄ and (OMe)₄: alkoxides such as butoxyl and methoxyl groups;and

Pr: a propyl group.

The oxygen ionic conductivity of the ionic electrolyte membranestructure obtained was measured with impedance analyzer (Solatron-1260)according to the standard measuring method: JIS-R-1661 to find that ahigh ionic conductivity of 1×10 S/cm was obtained at T=200° C.

Example 2

An ionic electrolyte membrane structure according to Example 2 wasobtained in the same way as in Example 1 except that the YSZ used as theion conductive material in Example 1 was changed for LaGaO₃.

The raw-material gases for the ion conductive material and ionnon-conductive material and the purging gases as used in the ALD areshown in Table 2 below.

TABLE 2 Reaction Purging Reaction Purging product A gas product B gasLaGaO₃ La (thd)₃ N₂ GaCl₃ O₂ SrTiO₃ Sr (CpPr₃)₂ N₂ Ti (OMe)₄ N₂(thd)₃: tri-tetramethyl-heptanedionate.

The oxygen ionic conductivity of the ionic electrolyte membranestructure obtained was measured to find that a high ionic conductivityof 5×10⁻¹ S/cm was obtained at T=200° C.

Example 3

An ionic electrolyte membrane structure according to

Example 3 was obtained in the same way as in Example 1 except that theYSZ used as the ion conductive material in Example 1 was changed forCeO₂.

The raw-material gases for the ion conductive material and ionnon-conductive material and the purging gases as used in the ALD areshown in Table 3 below.

TABLE 3 Reaction Purging Reaction Purging product A gas product B gasCeO₂ Ce (thd)₄ N₂ O₂ N₂ SrTiO₃ Sr (CpPr₃)₂ N₂ Ti (OMe)₄ N₂(thd)₄: tetra-tetramethyl-heptanedionate.

The oxygen ionic conductivity of the ionic electrolyte membranestructure obtained was measured to find that a high ionic conductivityof 2×10⁻¹ S/cm was obtained at T=200° C.

Example 4

A 30 μm thick alumina substrate obtained by anodizing an aluminum plateand having pores (100 nm in diameter) as through holes in the thicknessdirection of the substrate, standing open over the whole surfacethereof, was used as a substrate of an ionic electrolyte membranestructure according to Example 4. Then, BaCeO₃ was used as an ionconductive material and STO (SrTiO₃) as an ion non-conductive material,and the BaCeO₃ and the STO were alternately deposited in laminae by ADLon each inner wall surface of the pores in such a way that the layerswere each 5 nm in thickness and the multi-layer membranes thus formedfilled up the pores completely.

Stated more specifically, the alumina substrate was fastened onto astand with a heater, placed in a vacuum chamber, and the aluminasubstrate was kept at 250° C. while evacuating the inside of the vacuumchamber.

Except using the above materials, the subsequent procedure of Example 1was repeated by the ALD and under the like conditions.

The raw-material gases for the ion conductive material and ionnon-conductive material and the purging gases as used in the ALD areshown in Table 4 below.

TABLE 4 Reaction Purging Reaction Purging product A gas product B gasBaCeO₃ Ba (OEt)₃ N₂ Ce (thd)₄ O₂ SrTiO₃ Sr (CpPr₃)₂ N₂ Ti (OMe)₄ N₂(OEt)₃: an ethoxyl group.

The proton (hydrogen ion) ionic conductivity of the ionic electrolytemembrane structure obtained was measured to find that a value ofσ=1×10⁻² S/cm was obtained at T=650° C.

Example 5

An ionic electrolyte membrane structure according to Example 5 wasobtained in the same way as in Example 4 except that the BaCeO₃ used asthe ion conductive material in Example 4 was changed for SrCeO₃.

The raw-material gases for the ion conductive material and ionnon-conductive material and the purging gases as used in the ALD areshown in Table 5 below.

TABLE 5 Reaction Purging Reaction Purging product A gas product B gasSrCeO₃ Sr (CpPr₃)₂ N₂ Ce (thd)₄ O₂ SrTiO₃ Sr (CpPr₃)₂ N₂ Ti (OMe)₄ N₂(CpPr₃)₂: Cp: a cyclopentadienyl group, Pr: a propyl group.

The proton (hydrogen ion) ionic conductivity of the ionic electrolytemembrane structure obtained was measured to find that a value ofσ=8×10⁻³ S/cm was obtained at T=650° C.

Example 6

An ionic electrolyte membrane structure according to Example 6 wasobtained in the same way as in Example 4 except that the BaCeO₃ used asthe ion conductive material in Example 4 was changed for BaZrO₃.

The raw-material gases for the ion conductive material and ionnon-conductive material and the purging gases as used in the ALD areshown in Table 6 below.

TABLE 6 Reaction Purging Reaction Purging product A gas product B gasBaZrO₃ Ba (OEt)₃ N₂ ZrCl₄ N₂ SrTiO₃ Sr (CpPr₃)₂ N₂ Ti (OMe)₄ N₂

The proton (hydrogen ion) ionic conductivity of the ionic electrolytemembrane structure obtained was measured to find that a large value ofσ=9×10⁻³ S/cm was shown at T=650° C.

Example 7

An ionic electrolyte membrane structure according to Example 7 wasobtained in the same way as in Example 4 except that the BaCeO₃ used asthe ion conductive material in Example 4 was changed for CeO₂.

The raw-material gases for the ion conductive material and ionnon-conductive material and the purging gases as used in the ALD areshown in Table 7 below.

TABLE 7 Reaction Purging Reaction Purging product A gas product B gasCeO₂ Ce (thd)₄ N₂ O₂ N₂ SrTiO₃ Sr (CpPr₃)₂ N₂ Ti (OMe)₄ N₂

The proton (hydrogen ion) ionic conductivity of the ionic electrolytemembrane structure obtained was measured to find that a value ofσ=4×10⁻³ S/cm was shown at T=650° C.

Example 8

To one side of the substrate in the ionic electrolyte membrane structureaccording to Example 1 as shown in FIG. 7, a Pt thin plate which was toserve as an electrode was bonded with a silver paste. Then, thesubstrate was, on the other side thereof, slightly scratched withsandpaper, and thereafter the alumina portion (substrate portion) wasremoved by dissolution using a chromium-phosphoric acid liquid mixtureto leave columnar multi-layer membranes to provide between the columnarmulti-layer membranes an empty space extending through in the thicknessdirection of the structure to be formed (i.e., an empty space formedbetween the columnar multi-layer membranes on the Pt thin plate afterthe alumina portion was removed as shown in FIG. 8).

Next, in the same way as in Example 1 that the YSZ and the STO werealternately deposited in laminae by ADL on each inner wall surface ofthe pores, YSZ and STO were further alternately deposited in laminae onthe inner wall surface of the empty space defined substantially by thecolumnar multi-layer membranes, where their deposition was stopped atthe time the multi-layer membranes came to fill up the empty spacecompletely. Thus, it was able to form ionic electrolyte membranes overthe whole area of the structure.

The oxygen ionic conductivity of the ionic electrolyte membranestructure obtained was measured to find that a large value of σ=7×10¹S/cm was shown at T=200° C.

Example 9

The ionic electrolyte membrane structure produced in Example 8 was usedto produce an SOFC electrolyte layer.

Stated specifically, the back surface of the above ionic electrolytemembrane structure was partly removed by rubbing with sandpaper.Thereafter, an NiO porous substrate was used as the anode and an SmCoO³oxide as the cathode, and the material member obtained was baked at 500°C. to produce a solid oxide fuel cell (SOFC).

Next, hydrogen was fed to the fuel pole side and air to the oxygen poleside to test the generation of electricity at 200° C. Electricitygeneration characteristics were measured by a four-terminal methodaccording to JIS-R-1661.

Voltage, current density and power density measured thereby were asshown in FIG. 10.

POSSIBILITY OF INDUSTRIAL APPLICATION

The ionic electrolyte membrane structure according to the presentinvention is so structured that the edge faces of the interfaces betweenion conducting layers and ion non-conducting layers which havealternately been formed in laminae on the pore inner wall surfaces or onthe previous pore inner wall surfaces and the inner wall surface of theempty space defined substantially by the columnar multi-layer membranesstand bare on a plane, where the ions are transmitted in the throughdirection of the pores which have been made through the substrate or ofthe pores which had been made through the substrate and the space whichhad extended through between the columnar multi-layer membranes. Hence,the edge faces of the interfaces between ion conducting layers and ionnon-conducting layers can be made into contact with the air pole and thefuel pole. Thus, it has a possibility of industrial application that itis usable as the solid electrolyte of a solid oxide fuel cell (SOFC)having a superior energy efficiency.

1. An ionic electrolyte membrane structure which transmits ions only,comprising: a substrate having a plurality of pores which have been madethrough the substrate in the thickness direction thereof; and aplurality of multi-layer membranes each comprising an ion conductinglayer formed of an ion conductive material and an ion non-conductinglayer formed of an ion non-conductive material which have alternatelybeen formed in laminae a plurality of times on each inner wall surfaceof the pores of the substrate in such a way that the multi-layermembranes fill up the pores completely; the ions only being transmittedin the through direction by way of the multi-layer membranes provided onthe inner wall surfaces of the pores.
 2. An ionic electrolyte membranestructure which transmits ions only, comprising: a united multi-layermembrane comprising an ion conducting layer formed of an ion conductivematerial and an ion non-conducting layer formed of an ion non-conductivematerial which have alternately been formed in laminae a plurality oftimes; the united multi-layer membrane having been formed by: bonding asubstrate on one side thereof to a holding plate; the substrate being asubstrate in an ionic electrolyte membrane structure comprising i) asubstrate having a plurality of pores which have been made through thesubstrate in the thickness direction thereof and ii) a plurality ofmulti-layer membranes each comprising an ion conducting layer formed ofan ion conductive material and an ion non-conducting layer formed of anion non-conductive material which have alternately been formed inlaminae a plurality of times on each inner wall surface of the pores ofthe substrate in such a way that the multi-layer membranes fill up thepores completely; removing the substrate by dissolution to leavecolumnar multi-layer membranes to provide between the columnarmulti-layer membranes an empty space extending through in the thicknessdirection of the structure to be formed; and alternately forming inlaminae a plurality of times an ion conducting layer formed of an ionconductive material and an ion non-conducting layer formed of an ionnon-conductive material on the inner wall surface of the empty spacedefined substantially by the columnar multi-layer membranes, serving asfirst multi-layer membranes, to form a second multi-layer membrane whichfills up the empty space completely; the first multi-layer membranes andsecond multi-layer membrane having been united to form the unitedmulti-layer membrane; the ions only being transmitted in the throughdirection by way of the united multi-layer membrane formed.
 3. The ionicelectrolyte membrane structure according to claim 1 or 2, wherein eachlayer of the ion conducting layer and ion non-conducting layer is in alayer thickness of from 1 atomic layer or more to 10 nm or less.
 4. Theionic electrolyte membrane structure according to claim 1 or 2, whereinthe ions to be transmitted in the through direction by way of themulti-layer membranes or united multi-layer membrane formed are oxygenions.
 5. The ionic electrolyte membrane structure according to claim 1or 2, wherein the ion conducting layer is formed of an oxygen ionconductive material, and the oxygen ion conductive material is at leastone selected from YSZ, LaGaO₃, CeO₂, SrFeO₃, and SrCoO_(3-x).
 6. Theionic electrolyte membrane structure according to claim 1 or 2, whereinthe ions to be transmitted in the through direction by way of themulti-layer membranes or united multi-layer membrane formed are hydrogenions.
 7. The ionic electrolyte membrane structure according to claim 1or 2, wherein the ion conducting layer is formed of a hydrogen ionconductive material having perovskite structure.
 8. The ionicelectrolyte membrane structure according to claim 7, wherein thehydrogen ion conductive material having perovskite structure is at leastone material selected from BaCeO₃, SrCeO₃, BaZrO₃ and CeO₂.
 9. A methodfor producing an ionic electrolyte membrane structure, comprising: oneach inner wall surface of pores of a substrate having a plurality ofpores which have been made through the substrate in the thicknessdirection thereof, alternately forming in laminae a plurality of timesan ion conducting layer formed of an ion conductive material and an ionnon-conducting layer formed of an ion non-conductive material, by atomiclayer deposition to form a plurality of multi-layer membranes which fillup the pores completely.
 10. A method for producing an ionic electrolytemembrane structure, comprising: bonding a substrate on one side thereofto a holding plate; the substrate being a substrate in an ionicelectrolyte membrane structure comprising i) a substrate having aplurality of pores which have been made through the substrate in thethickness direction thereof and ii) a plurality of multi-layer membraneseach comprising an ion conducting layer formed of an ion conductivematerial and an ion non-conducting layer formed of an ion non-conductivematerial which have alternately been formed in laminae a plurality oftimes on each inner wall surface of the pores of the substrate in such away that the multi-layer membranes fill up the pores completely;removing the substrate by dissolution to leave columnar multi-layermembranes to provide between the columnar multi-layer membranes an emptyspace extending through in the thickness direction of the structure tobe formed; and alternately forming in laminae a plurality of times anion conducting layer formed of an ion conductive material and an ionnon-conducting layer formed of an ion non-conductive material, by atomiclayer deposition on the inner wall surface of the empty space definedsubstantially by the columnar multi-layer membranes, serving as firstmulti-layer membranes, to form a second multi-layer membrane which fillsup the empty space completely; the first multi-layer membranes andsecond multi-layer membrane having been united to form a unitedmulti-layer membrane.
 11. A solid oxide fuel cell comprising a solidelectrolyte capable of transmitting ions selectively, and an air poleprovided on one side of the solid electrolyte and a fuel pole providedon the other side thereof, wherein; the solid electrolyte comprises anionic electrolyte membrane structure which transmits ions only,comprising: a substrate having a plurality of pores which have been madethrough the substrate in the thickness direction thereof; and aplurality of multi-layer membranes each comprising an ion conductinglayer formed of an ion conductive material and an ion non-conductinglayer formed of an ion non-conductive material which have alternatelybeen formed in laminae a plurality of times on each inner wall surfaceof the pores of the substrate in such a way that the multi-layermembranes fill up the pores completely; the ions only being transmittedin the through direction by way of the multi-layer membranes provided onthe inner wall surfaces of the pores; or an ionic electrolyte membranestructure which transmits ions only, comprising: a united multi-layermembrane comprising an ion conducting layer formed of an ion conductivematerial and an ion non-conducting layer formed of an ion non-conductivematerial which have alternately been formed in laminae a plurality oftimes; the united multi-layer membrane having been formed by: bonding asubstrate on one side thereof to a holding plate; the substrate being asubstrate in an ionic electrolyte membrane structure comprising i) asubstrate having a plurality of pores which have been made through thesubstrate in the thickness direction thereof and ii) a plurality ofmulti-layer membranes each comprising an ion conducting layer formed ofan ion conductive material and an ion non-conducting layer formed of anion non-conductive material which have alternately been formed inlaminae a plurality of times on each inner wall surface of the pores ofthe substrate in such a way that the multi-layer membranes fill up thepores completely; removing the substrate by dissolution to leavecolumnar multi-layer membranes to provide between the columnarmulti-layer membranes an empty space extending through in the thicknessdirection of the structure to be formed; and alternately forming inlaminae a plurality of times an ion conducting layer formed of an ionconductive material and an ion non-conducting layer formed of an ionnon-conductive material on the inner wall surface of the empty spacedefined substantially by the columnar multi-layer membranes, serving asfirst multi-layer membranes, to form a second multi-layer membrane whichfills up the empty space completely; the first multi-layer membranes andsecond multi-layer membrane having been united to form the unitedmulti-layer membrane; the ions only being transmitted in the throughdirection by way of the united multi-layer membrane formed.